1. Field of the Invention
This invention relates to a method for depositing a barrier coating to plastic containers for improving the effective barrier of the plastic container against gas and water permeability.
2. Description of the Related Art
In numerous fields of technology, it is necessary to apply very thin coatings of pure substances to certain objects. An example is window glass which is provided with a thin coating of metal oxide in order to filter certain wavelength ranges out of sunlight. In semi-conductor technology, thin coatings of one or more substances are often applied to a substrate. It is especially important that the thin coatings not only be pure, but also that they be precisely measured out so that the coating thickness and, in the case of coatings of chemical compounds, their composition will be accurately repeatable. These coating thicknesses are, as a rule, between two and several thousands of nanometers.
A variety of methods are known for applying thin coatings to films, glass and other substrates. Such processes for depositing SiO.sub.x onto plastic objects are disclosed in U.S. Pat. No. 5,053,244 and European Patent No. 0 299 754. Most specifically, these processes can provide excellent barrier properties to plastic films which have not been exposed to dust. However, these processes provide only minor barrier properties to three-dimensional plastic objects or films that are exposed to dust.
It is believed that the inability to obtain a good barrier on three-dimensional plastic objects is due to the lack of cleanliness of the object's surface since most three dimension objects are exposed to dust during fabrication.
In the process of depositing SiO.sub.x on thin films, the SiO.sub.x barrier coating is applied under vacuum conditions to a clean film. Typically, film is extruded under very clean conditions and immediately wound into a large roll. As a consequence, the film surfaces, with the exception of the outside layer, are never exposed to particles in air such as dust.
It is believed that the reason for the lack or minimum improvement in permeability of three-dimensional objects coated with SiO.sub.x is that the surface of the three-dimensional object has a contaminated surface. It is further believed that the contamination is due to foreign surface particles that settle on the object due to its exposure to air.
Even though SiO.sub.x is evenly deposited on the surface of an object at about 500 to 1000 .ANG. in thickness, because foreign surface particles, that are on the order of 5000 to 50000 .ANG. in diameter, may be on the surface of the object, portions of the surface are not coated with the SiO.sub.x because of the shielding effect caused by the foreign surface particles.
Therefore, a need exists to remove and/or redistribute contamination from the surface of objects that are to be coated with SiO.sub.x to improve the process for applying SiO.sub.x to the objects and more particularly, to improve the barrier properties of the objects.